1. Field of the Invention
This invention generally relates to Analog to Digital (“A/D”) conversion. More specifically, the invention relates to a plurality of A/D conversions in which at least one of the A/D conversions is operable with a signal cancellation system of a receiver. Such a system may be particularly beneficial to communications employing Code Division Multiple Access (“CDMA”), Wideband CDMA, Broadband CDMA, (“UMTS”), Global Positioning System (“GPS”) and combinations thereof.
2. Discussion of the Related Art
Interference in communications obstructs the intended reception of a signal and is a persistent problem. Interference may exist in many forms. In CDMA communications, for example, interference is typically the result of receiving one or more unwanted signals simultaneously with a selected signal. These unwanted signals may be similar to that of the selected signal and may therefore disrupt the reception of the selected signal. This disruption of the selected signal may corrupt data retrieval processes of a selected signal. Such problems are typical in CDMA telephony systems.
In CDMA telephony, a communications system typically includes a plurality of “base station” transceivers providing a coverage area within a geographic region. These base stations communicate with mobile telephones and/or other CDMA devices operating within the coverage area. To illustrate, a base station provides a coverage “cell” within the overall communication coverage area maintained by the communications system. While within a particular cell, a mobile telephone can communicate with the base station providing the coverage for that cell. As the mobile telephone moves to the cell of another base station, communications between the mobile telephone and the base station providing the initial cell coverage can be transferred via a “hand off” to the other base station. Typically, CDMA cells have overlapping coverage.
Each base station within a CDMA telephony system uses coded signals to communicate with mobile telephones. For example, typical CDMA telephony systems use pseudorandom number, or pseudo-noise, (“PN”) spreading codes, occasionally referred to as “short codes,” to encode data signals. These encoded data signals are transmitted to and from mobile telephones to convey digitized voice and/or other forms of digital communication. PN codes are known to those skilled in the art.
To encode the data signals, the base station applies a short code to the data at a rate that is faster than that of the symbol rate. For example, the short code is applied to the data such that there are multiple “chips” of the code for any given symbol of data. Such an application of the short code is commonly referred to as direct sequence spreading of the data. Chips and their associated chipping rates are known to those skilled in the art.
Often, each base station is assigned a particular timing offset of the short code to differentiate between base stations. Mobile telephones may therefore determine the identity of a particular base station based on the timing offset of the short code. Additionally, the data signals are often further encoded with a unique “covering” code. Such covering codes provide “channelization” for a signal that increases the probability of data recovery of a selected signal. For example, data encoded with a covering code can further differentiate signals thereby improving detection and subsequent processing of a selected signal.
These covering codes are often used in CDMA telephony systems and typically include families of codes that are orthogonal (e.g., a Walsh code) or codes that are substantially orthogonal (e.g., a quasi-orthogonal function (“QOF”)). Orthogonal and substantially orthogonal covering codes have properties that allow for the differentiation of unwanted signals and are known to those skilled in the art. Walsh codes and QOF codes are known to those skilled in the art.
Both the short codes and the covering codes assist in the detection, acquisition, tracking and data recovery of a selected signal. However, interference caused by other signals may still degrade data recovery of the selected signal. For example, as a mobile telephone communicates with a particular base station within the coverage cell of that base station, communication to and from other base stations can interfere with the mobile telephone communication. Since cells often overlap one another to ensure that all desired geographic regions are included in the coverage area of the communication system, one or more signals to and from one base station may interfere with the communication link, or “channel,” between the mobile telephone and another base station. This effect is commonly referred to as cross-channel interference.
Still, other forms of interference may occur from “multipath” copies of a selected signal. Multipath can create interference due to the reception of multiple copies of a selected signal at differing times. Multipath typically occurs because of obstructions, such as buildings, trees, et cetera, that create multiple transmission paths for a selected signal. These separate transmission paths may have unique distances that cause the signal to arrive at a receiver at differing times and is commonly referred to as co-channel interference. Additionally, the effects of these separate paths may bleed over into other cells to cause cross-channel interference.
Multipath creates co-channel interference because, among other reasons, the orthogonality of the covering codes for a received signal is essentially lost due to timing offsets associated with the multipath. For example, a multipath signals arriving at a receiver at differing times cause a misalignment of the covering code. Such a misalignment can result in a high cross-correlation between covering codes and a reduction in the ability to correctly retrieve conveyed data.
“Rake” receivers, such as those used in CDMA telephony systems, can assist in countering interfering effects caused by multipath. For example, a rake receiver may have a plurality of “fingers,” wherein each finger of the rake receiver independently estimates channel gain and other signal characteristics (e.g., phase) of the selected signal to more accurately demodulate data of the selected signal and subsequently retrieve the data. Each finger is assigned a particular “path” of the selected signal (i.e., one of the paths of a transmitted signal that comprises multipath signals). Additionally, as signal characteristics change, the fingers may be assigned or de-assigned to other “paths” of the signal to improve data retrieval.
FIG. 1 illustrates a block diagram of a prior art rake receiver 100. Rake receiver 100 is configured for receiving an analog radio signal and converting that radio signal to a digital signal in the receiver front-end 110. The digital signal is subsequently processed by the rake receiver (i.e., by among other things processing fingers 1041 . . . P, where “P” is an integer greater than one) to extract data and/or voice from the digital signal.
The digital signal is transferred to searcher finger 103, which detects a signal path that is subsequently assigned to a processing finger 104 for tracking and demodulation. Each processing finger 104 is assigned a unique signal path to process the signal of interest (“SOI”) of the received signal. For example, each processing finger 104 may track a signal path that comprises a plurality of channels. The processing finger may then demodulate a particular channel for subsequent combining and processing.
Combiner 105 combines the processed signals from processing fingers 104 to improve an estimate of the SOI. For example, combiner 105 may perform maximal ratio combining (“MRC”) of the demodulated signals produced by each of the processing fingers 104. This combining may improve the signal quality of the SOI estimate and is subsequently processed by the rake receiver 100 to extract data and/or voice. Extraction of the data and/or voice is performed by descrambler 106, deinterleaver 107 and decoder 108 of rake receiver 100. Such descrambling, deinterleaving and decoding is well-known to those skilled in the art.
Prior to descrambling, deinterleaving and decoding of the SOI, rake receiver 100 converts the received analog radio signal to a digital signal. Rake receiver 100 uses a receiver front-end 110 that comprises a downconverter 101 and discretizer 102 to convert the received radio signal to a digital signal. Downconverter 101 receives the analog radio signal, down converts that signal to a baseband signal and low pass filters the baseband signal. The filtered baseband signal is transferred to discretizer 102 for A/D conversion of the baseband signal and subsequent amplitude adjustment and scaling of the resultant digital signal. Upon conversion of the radio signal by receiver front-end 110, the resultant digital signal is transferred to searcher finger 103 and/or processing finger 104 for the processing described hereinabove.
FIG. 2 illustrates a block diagram of prior art rake receiver front-end 110. Rake receiver front-end 110 comprises down converter 101 and discretizer 102 of FIG. 1. The prior art receiver front-end 110 is configured for receiving an analog radio signal and down converting and discretizing that signal to a baseband digital signal.
Down converter 101 comprises baseband converter 201 and low pass filter (“LPF”) 202. Baseband down converter 201 receives the analog radio signal and down converts the signal to baseband as is well known to those skilled in the art. Also well-known to those skilled in the art is the subsequent low pass filtering (i.e., by LPF 202) of the baseband signal. The low pass filtered baseband signal is transferred to discretizer 102 wherein the signal is digitized by A/D converter 203.
Once the signal is digitized by A/D converter 203, the digital signal is transferred to a gain controller 204 for amplitude adjustment and scaling. Typically, the gain controller is an Automatic Gain Controller (“AGC”) that receives the digital signal and automatically adjusts the amplitude of the signal based on a requisite dynamic range of the receiver. The amplitude adjusted signal is then transferred to a searcher finger and/or a processing finger, such as searcher finger 103 and processing finger 104 of FIG. 1, for further processing as described hereinabove. A/D converters and gain controllers (e.g., AGCs) are well-known to those skilled in the art.
The allocated number of bits (i.e., bit width) produced in the AGC of prior art rake receivers as a representation of the received signal is a trade-off between accuracy in data recovery and processing limitations. These rake receivers are typically not concerned with interference cancellation that may require an increased number of bits and as such do not produce a second digital representation of the signal with greater bit accuracy.